In an ink jet printing apparatus, generally, not all print data use all the plurality of nozzles provided on a printing head, and there is a case where a particular nozzle is not used for a long time. Water or solvent in such a nozzle evaporates, and thus the viscosity thereof increases. As a result, in some cases, ink is not ejected properly from such a nozzle, even when a drive signal is applied to an element generating energy used for ejecting the ink. This causes a deflection of the ejecting direction, and allows only an insufficient amount of ink to be ejected, or in extreme cases, ink is not ejected at all (hereinafter, these poor conditions are called an ejection failure). As a result, a desired image cannot sometimes be obtained.
An operation called a preliminary ejection is performed as one of treatments for eliminating the factors causing the ejection failure and for restoring a favorable ink ejection performance of a printing head. In this operation, ink is ejected by driving an element generating energy used for ejecting the ink, for the purpose of refreshing the ink in a nozzle, in addition to the purpose of forming an image in a printing operation. In particular, this operation is aimed at ejecting, from a nozzle, ink in an inadequate condition to secure the ejection performance and printing quality when the partial evaporation of volatile constituents such as water or solvent contained in the ink results in the inadequate condition.
Conventionally, a certain arbitrarily chosen method has been used in order to perform the preliminary ejection. For example, the preliminary ejection is performed in a state where the printing head faces a cap provided outside a printing area of the printing head. In this case, however, the printing head is required to move away from the printing area, but this movement needs a longer time of suspending the printing operation. As a result, the throughput of printing decreases.
In contrast, there is a method in which the preliminary ejection operation is performed by ejecting thickened ink in nozzles directly onto a printing medium without suspending the printing operation (for example, Japanese Patent Application Laid-Open No. 6-40042 (1994) and Japanese Patent Application Laid-Open No. 55-139269 (1980)). In this preliminary ejection operation, ink is ejected onto a printing medium such as a sheet of paper. This preliminary ejection has an advantage of avoiding a decrease in the throughput of printing, since the preliminary ejection operation can be performed during the printing operation.
In this preliminary ejection operation, ink is ejected directly onto a printing medium on which an image is actually printed. Accordingly, when an ink dot with a certain size and a certain density is formed on the printing medium with the ink of the preliminary ejection, the ink dot may be noticeable, and may deteriorate the printing quality. There is a method to solve this problem by making the dot formed with the ink of the preliminary ejection less noticeable. To make the dot less noticeable, the ink is ejected onto an area with a high optical reflection density, such as a black letter, in the image.
Nevertheless, in the circumstance in which various kinds of printing are performed to meet the demands of users as the use of ink jet printing apparatuses is increasing, a printing image does not necessarily include an area with a high optical reflection density, such as a black letter. In this case, the preliminary ejection of ink has to be done onto a color image on a printing medium.
The inventors of the present invention, however, found that the printing quality is deteriorated in a case where the color image and the ink that is ejected preliminarily have a certain relationship. In addition, the present inventors also found that the deterioration in the printing quality is particularly remarkable in a case where a dot of low lightness is formed by preliminarily ejecting ink which presents relatively low lightness on a printing medium, such as a cyan ink, on an image area formed with ink which presents relatively high lightness on a printing medium, such as a yellow ink.
This finding will be explained by using FIGS. 12A to 12D. First, assume that there is a yellow (Y) ink image data covering an area extending from the coordinates n to n+2 in a main-scan direction and from the coordinates m to m+1 in a sub-scan direction, as shown in FIG. 12A. Then, assume that a cyan (C) ink is to be preliminarily ejected onto the set of coordinates (n+1, m+1), as shown in FIG. 12B. As a result, a cyan ink dot is formed in the set of coordinates (n+1, m+1) on the printing medium, overlapping a yellow ink dot, as shown in FIG. 12C. In this position, the dot has a green color that is a secondary color made by the yellow ink dot and the cyan ink dot.
However, a problem arises here. The problem is that the cyan ink dot, which is landed on the set of coordinates (n+1, m+1), expands widely (to an area circled with a dashed line), as shown in FIG. 12D. To be more precise, the cyan ink dot (hereinafter called “a later-landed dot”) overlaps the yellow ink dot (hereinafter called “an earlier-landed dot”), and expands, thereby forming a dot with a large diameter. In other words, the later-landed cyan ink dot expands into areas of the yellow ink dots formed in other sets of coordinates surrounding the set of coordinates where the cyan ink dot is landed.
The lightness of the yellow area formed on the printing medium is high, and the area is light and bright to the human eye. For this reason, if the large dot of low lightness exists in the yellow area, the contrast between the large dot of low lightness and the yellow color background becomes very noticeable, and thus the visual detectability thereof is increased. This results in the deterioration in the printing quality.
This problem arises not only in the relationship between yellow and cyan, but also in a case where a dark color ink, such as a magenta ink or a black ink, is preliminarily ejected onto an image area formed with the yellow ink. In addition, nowadays, there is a printing apparatus using a light cyan ink and a light magenta ink. This problem also arises in a case where an ink of low lightness is preliminarily ejected onto an area printed with any of these light color inks. In other words, when an ink of low lightness is preliminarily ejected onto an image area formed of ink dots of high lightness, the dot of low lightness that is preliminarily ejected becomes very noticeable.